Conventionally, in a client/server system that performs authentication on a communication network, user identification has been widely used by means of BASIC authentication, which transmits authentication information such as a user name, and password; and DIGEST authentication, which transmits the authentication information having been encrypted. The technology described in Japanese Patent Unexamined Publication No. 2002-269052 implements more strict authentication by means of the combination of a mobile phone number, the terminal-specific ID number, and the user's fingerprint. Meanwhile, Web-enabled TV (referred to as “net TV” hereinafter) performs authentication by transmitting a unique terminal ID according to a device-specific value, to a server. The software in a television receiver for digital broadcasting can be upgraded by being rewritten using on-air downloading. The software in a net TV with digital broadcasting reception can be upgraded by downloading as well.